Orthodontic palatal expansion is commonly carried out using a force module that is installed on the upper dental arch of a patient's mouth, the force module being connected to a palatal expander of a type commonly known in the art. The method of palatal expansion using expansion screws has become known in the art as “rapid palatal expansion” (RPE). Force modules used for this purpose must be tightened periodically to be effective.
Huge et al. U.S. Pat. No. 6,783,361, Aug. 31, 2004, describes an orthodontic mechanical force module having a jack screw whose rotation is controlled by a ratchet. In one embodiment, a pair of housings are interconnected with the patient in any appropriate manner, and are further rotatably interconnected with a rotatable spindle. At least one of these housings is also threadedly engaged with the spindle. A ratchet is associated with the spindle to selectively allow the spindle to rotate only in a direction that increases a magnitude of the treatment forces being exerted on the patient by changing the spacing between the pair of housings by a movement of at least one of the housings along the spindle due to its rotation. In an active position, the ratchet precludes the spindle from rotating in a direction that would tend to reduce the magnitude of the treatment forces being exerted on the patient.
The Huge et al. device, and also an appliance described in literature as the “RatchetRax” claim to provide bi-directional adjustment capability. The device of the present invention incorporates a mechanical locking device, unlike those of the previously mentioned devices, that allows bi-directional activation of the screw-mechanism without a multitude of tools or augmented instruction to the user. The RatchetRax is bi-directional, but moves only one nut body to create the expansion. The great disadvantage inherent with movement of one nut body as opposed to both nut bodies is that the overall lateral size of the single body movement device must necessarily be twice as wide to effect the same amount of expansion or contraction. Appliance size is critical relative to accommodation in the mouth and to patient comfort and tolerance.
Bi-directional adjustability allows the mechanism to be employed not only as an expansion device (activation of the nut bodies apart from each other) but also as a contraction device wherein the bodies are purposefully installed in the patient screwed apart and then activated to bring them together, thereby delivering a closing force as opposed to an expanding or opening force. This is a common application of such force modules, i.e. to move teeth adjacent to an edentulous space together to eliminate the space and mitigate the need for a prosthesis. The most common applications of such force modules are expansion of the palate, expansion of the mandibular alveolus, contraction of teeth adjacent to an edentulous site, and contraction of the palate.
Screw de-activation (screw reverses direction after activation) has always been a challenge with expansion screws. In order to control screw reversal, most prior art mechanisms have incorporated some type of screw thread drag adjunct (lacquer coating, nylon bushing, etc.). These have sometimes been successful, but many times not. A popular device described in U.S. Pat. No. 6,482,001 suffered problems with screw reversal that could not be controlled. When an expansion device fails, its removal and replacement subjects the patient to the rigors of duplicating previous protocols. Additionally, after such expansion has begun and then fails, appliance removal is considerably more uncomfortable because the involved tissues are in a heightened state of irritability. Furthermore, clinicians experience aggravation over lost time, duplication of effort, and poor public relations that invariably result from patients and/or parents of patients exasperated about the inconvenience of repeated appointments and delayed treatment time. Locking the screw in place mechanically is the surest way to prevent screw reversal. The present invention inhibits screw reversal without requiring additional tooling and instruction.